Hydromagnetics of the magnetosphere

Different models of the magnetosphere are discussed critically. It is pointed out that there is a principal difference between the case when the impinging interplanetary plasma has no initial magnetization, B ₀ = 0, (as in the Chapman-Ferraro theory), and the case when the plasma is initially magnetized, B ₀ 0, even if B ₀ is very small.In the former case the plasma remains unmagnetized (like a superconductor) and cannot penetrate into the magnetosphere. Therefore the plasma is separated by a sharp boundary from the magnetosphere, (closed magnetosphere model).In the latter case when the plasma is magnetized (which is more realistic) there is a possibility that field lines run from the earth to infinity (open magnetosphere model). Particles from the interplanetary space may penetrate into the magnetosphere. At the same time there may be a number of discontinuity surfaces of different character, such as the Cahill discontinuity.It is important to make terrella experiments in order to study the complicated phenomena occurring when a magnetized plasma penetrates into a dipole field.     

Heavy ions in the magnetosphere

For purposes of this review heavy ions include all species of ions having a mass per unit charge of 2 AMU or greater. The discussion is limited primarily to ions in the energy range between 100 eV and 100 keV. Prior to the discovery in 1972 of large fluxes of energetic O⁺ ions precipitating into the auroral zone during geomagnetic storms, the only reported magnetosphere ion species observed in this energy range were helium and hydrogen. More recently O⁺ and He⁺ have been identified as significant components of the storm time ring current, suggesting that an ionosphere source may be involved in the generation of the fluxes responsible for this current. Mass spectrometer measurements on board the S3-3 satellite have shown that ionospheric ions in the auroral zone are frequently accelerated upward along geomagnetic field lines to several keV energy in the altitude region from 5000 km to greater than 8000 km. These observations also show evidence for acceleration perpendicular to the magnetic field and thus cannot be explained by a parallel electric field alone. This auroral acceleration region is most likely the source for the magnetospheric heavy ions of ionospheric origin, but further acceleration would probably be required to bring them to characteristic ring current energies. Recent observations from the GEOS-1 spacecraft combined with earlier results suggest comparable contributions to the hot magnetopheric plasma from the solar wind and the ionosphere.Proceedings of the Symposium on Solar Terrestrial Physics held in Innsbruck, May–June 1978.     

Some processes in the magnetosphere

This paper is a continuation of the review (Shabansky, 1968), which is quoted here as I. This paper considers the problems related to the processes of the interaction of charged particles with the geomagnetic field, and also contains the original results obtained by the author (1, 2.1; 2.2; 2.3.4; 2.4; 3.1; 3.2; 3.3; 3.4.3). The problems under discussion in the Sections 1; 2.1; 2.2 were partly dealt with in the end of paper I. However, the equations of motion for the two-and three-component plasma shortly represented in 1.1, 1.2, take into account the inertial terms in distinction to paper I. The ionospheric current induction (1.4), the transition layer (2.1) and the magnetotail rotation (2.2) are considered in more detail than in I.The references are divided into 2 parts: the papers published in Soviet magazines are listed separately, using the Cyrillic alphabet.     

Hydromagnetic motions in the magnetosphere

Following a brief specification and historical review of hydromagnetic motions in the magnetosphere, the principles of the governing and limiting processes are surveyed. A formal proof of the well-known hydromagnetic theorem is included, and its interpretation in terms of frozen fields is discussed. Some consequences of its application to the magnetosphere are then described, and the value of equipotentials as a means of illuminating the discussion is established. Departures from the hydromagnetic approximation are then evaluated, and their resultant currents described.The general principles find application in a number of processes: rotation, high-latitude circulation in quiet and disturbed conditions, more widespread convection under continuous dynamo action, and irregular motion both of an unstable and of a forced type. All these are reviewed, and one emergent point is emphasized: that direct evidence for the hydromagnetic motions is lacking, but that it can and should be sought.     

IMF control of the Earth's magnetosphere

We review recent progress in the understanding of the IMF control on the Earth's magnetosphere through the reconnection process. Major points include, (1) the identification of the magnetopause structure under the southward IMF polarity to be the rotational discontinuity and the resulting inference that the reconnection line is formed in the equatorial region, and (2) the confirmation from several observational aspects that under the northward IMF the reconnection takes place in the polar cusp. The point (1) is consistent with the observed correlations of geomagnetic indices with IMF but raises an important theoretical issue, and the point (2) is accompanied by an interesting issue of explaining why the polar cap electron precipitation is more energetic under such IMF conditions. Critical studies have reaffirmed the view that the energy supplied by reconnection is partly transported directly to the ionosphere to drive the DP-2 type current system but at the same time it is partly stored in the magnetic field of the tail to be unloaded 0.5 1 hr later to produce the expansion phase of substorm.Presented at the Fifth International Symposium on Solar-Terrestrial Physics, held at Ottawa, Canada, May 1982.     

Fluctuating magnetic fields in the magnetosphere

The study of ULF waves in space has been in progress for about 12 years. However, because of numerous observational difficulties the properties of the waves in this frequency band (10^-3 to 1 Hz) are poorly known. These difficulties include the nature of satellite orbits, telemetry limitations on magnetometer frequency response and compromises between dynamic range and resolution. Despite the paucity of information, there is increasing recognition of the importance of these measurements in magnetospheric processes. A number of recent theoretical papers point out the roles such waves play in the dynamic behavior of radiation belt particles.At the present time the existing satellite observations of ULF waves suggest that the level of geomagnetic activity controls the types of waves which occur within the magnetosphere. Consequently, we consider separately quiet times, times of magnetospheric substorms and times of magnetic storms. Within each of these categories there are distinctly different wave modes distinguished by their polarization: either transverse or parallel to the ambient field. In addition, these wave phenomena occur in distinct frequency bands. In terms of the standard nomenclature of ground micropulsation studies ULF wave types observed in the magnetosphere include quiet time transverse — Pc 1, Pc 3, Pc 4, Pc 5 quiet time compressional — Pc 1 and Pi 1; substorm compressional Pi 1 and Pi 2; storm transverse — Pc 1; storm compressional Pc 4, 5. The satellite observations are not yet sufficient to determine whether the various bands identified in the ground data are equally appropriate in space.Publication No. 982. Institute of Geophysics and Planetary Physics, University of California, Los Angeles, Calif. 90024.     

Fluctuating magnetic fields in the magnetosphere

The study of Extremely-Low-Frequency (ELF) and Very-Low-Frequency (VLF) waves in space has been intensively pursued in the past decade. Search coil magnetometers, magnetic loop antennas, and electric dipole antennas have been carried on board many spacecraft. The measurements performed by these instruments have revealed a multitude of wave phenomena, whose study in turn is providing a wealth of information on the physics of the magnetospheric and ionospheric plasma. Two classes of wave phenomena are observed: whistlers and emissions. The observed whistler phenomena include: multiple hop ducted whistlers, ion-cutoff whistlers, ion cyclotron whistlers, subprotonospheric whistlers, magnetospherically reflected whistlers and walking trace whistlers.The emissions observed at high altitudes near the magnetic equator differ in many respects from those observed at low altitudes near the ionosphere. At high altitudes, inside the plasmasphere ELF hiss is the dominant emission and outside the plasmasphere chorus is the dominant emission. Also seen is a sub-LHR hiss band in the outer plasmasphere near the equator, and high pass noise and broadband noise in the outer nightside magnetosphere. At low altitude both ELF hiss and chorus are present but, here, ELF hiss is the dominant emission even outside the plasmasphere. Additional emissions, specific to low altitudes, such as VLF hiss and LHR noise are also observed. Although the observations of these phenomena by spacecraft have been complemented by many ground-based and rocket borne studies as well as by spacecraft observations of man-made signals, this paper reviews only satellite observations of signals of natural origin.     

Large-scale electric field in the magnetosphere

A review is given on the distribution and origin of the large-scale electric field in the magnetosphere and its influence on the dynamical behavior of the magnetospheric plasma. Following a general discussion on the gross structure of the magnetosphere and its tail, two principal electric field systems are deduced from ground-based geomagnetic variations. One is responsible for the polar substorm, the DP 1 field, which is closely associated with the activation of the auroral electrojet. The other is responsible for the twin current vortices, the DP 2 field, and this represents the general convective system set up in the magnetospheric plasma.The origin of these magnetospheric electric fields is possibly resided in the domain of the solar wind interacting with the outer geomagnetic field. However, the mechanism, in which the energy is transferred, is still quite controversial. Several theories so far proposed are re-examined, and some modification of them are suggested to have a consistent understanding of these two types of electric fields. The effects of electric fields on magnetospheric plasma dynamics are described, such as the formation of the plasmapause, the acceleration and diffusion of energetic particles in the radiation belt.     

Interplanetary field effect on the magnetosphere

This paper reviews recent developments in the understanding of the solar-wind magnetosphere interaction process in which the interplanetary magnetic field has been found to play a key role. Extensive correlative studies between the interplanetary magnetic field and the magnetospheric parameters have in the past few years yielded detailed information on the nature of the interaction process and have made possible to follow the sequence of events that are produced inside the magnetosphere in consequence of the solar-wind energy transfer. We summarize the observed effects of the interplanetary magnetic field, its north-south and east-west components in particular, found in various domains of the magnetosphere — dayside magnetopause, polar cap, magnetotail, auroral zone —, and present an overall picture of the solar-wind magnetosphere interaction process. Dungey's reconnected magnetosphere model is used as a frame of reference and the basic compatibility of the observations with this model is emphasized. In order to avoid overlap with other review articles in the series discussion on the energy conversion process inside the magnetosphere leading to the substorm phenomenon is kept to the minimal.     

Stimulation of VLF amplification in the magnetosphere

Among the various plasma instabilities that exert influence on the dynamic equilibrium state of the magnetosphere, the cyclotron-resonance interaction appears to be the most accessible to artificial stimulation. The strength of the interaction is sensitive to both the background magnetoplasma parameters and the hot energetic particle distribution. Thus, proper modification of one or more conditions can induce significant wave amplification at the expense of hot plasma energy density. Several methods of hot and cold plasma injection have been investigated with the linear theory to assess their effectiveness as a means of stimulating amplification.Only the interaction of VLF waves (3–30 kHz) with hot electrons (0.1–100 keV) is treated here. The injection of a dense jet of barium that travels upward along the geomagnetic field causes appreciable amplification when the jet is within 30° of the geomagnetic equator. Injection of a geosynchronous lithium cloud stimulates amplification of both VLF and ULF waves, but the magnitude depends critically on the state of geomagnetic activity. Conventional hot electron beams may also amplify narrow frequency bands, but the net wave energy is severely limited by the beam energy.Although the cyclotron-resonance is recognized as a dominant interaction in magnetospheric dynamics, its properties have never been confirmed quantitatively by appropriate spacecraft experiments. Controlled injections would provide important insight into this fundamental process because the induced amplification has a well-defined signature.     

Theory of hydromagnetic waves in the magnetosphere

Space Science Reviews - Many of the significant theoretical advances in understanding the origin and behaviour of low frequency hydromagnetic waves originating in the magnetosphere in the last...     

Electrodynamic coupling of the magnetosphere and ionosphere

The auroral zone ionosphere is coupled to the outer magnetosphere by means of field-aligned currents. Parallel electric fields associated with these currents are now widely accepted to be responsible for the acceleration of auroral particles. This paper will review the theoretical concepts and models describing this coupling. The dynamics of auroral zone particles will be described, beginning with the adiabatic motions of particles in the converging geomagnetic field in the presence of parallel potential drops and then considering the modifications to these adiabatic trajectories due to wave-particle interactions. The formation of parallel electric fields can be viewed both from microscopic and macroscopic viewpoints. The presence of a current carrying plasma can give rise to plasma instabilities which in a weakly turbulent situation can affect the particle motions, giving rise to an effective resistivity in the plasma. Recent satellite observations, however, indicate that the parallel electric field is organized into discrete potential jumps, known as double layers. From a macroscopic viewpoint, the response of the particles to a parallel potential drop leads to an approximately linear relationship between the current density and the potential drop.The currents flowing in the auroral circuit must close in the ionosphere. To a first approximation, the ionospheric conductivity can be considered to be constant, and in this case combining the ionospheric Ohm's Law with the linear current-voltage relation for parallel currents leads to an outer scale length, above which electric fields can map down to the ionosphere and below which parallel electric fields become important. The effects of particle precipitation make the picture more complex, leading to enhanced ionization in upward current regions and to the possibility of feedback interactions with the magnetosphere.Determining adiabatic particle orbits in steady-state electric and magnetic fields can be used to determine the self-consistent particle and field distributions on auroral field lines. However, it is difficult to pursue this approach when the fields are varying with time. Magnetohydrodynamic (MHD) models deal with these time-dependent situations by treating the particles as a fluid. This class of model, however, cannot treat kinetic effects in detail. Such effects can in some cases be modeled by effective transport coefficients inserted into the MHD equations. Intrinsically time-dependent processes such as the development of magnetic micropulsations and the response of the magnetosphere to ionospheric fluctuations can be readily treated in this framework.The response of the lower altitude auroral zone depends in part on how the system is driven. Currents are generated in the outer parts of the magnetosphere as a result of the plasma convection. The dynamics of this region is in turn affected by the coupling to the ionosphere. Since dissipation rates are very low in the outer magnetosphere, the convection may become turbulent, implying that nonlinear effects such as spectral transfer of energy to different scales become important. MHD turbulence theory, modified by the ionospheric coupling, can describe the dynamics of the boundary-layer region. Turbulent MHD fluids can give rise to the generation of field-aligned currents through the so-called -effect, which is utilized in the theory of the generation of the Earth's magnetic field. It is suggested that similar processes acting in the boundary-layer plasma may be ultimately responsible for the generation of auroral currents.     

Plasma processes within the magnetosphere boundaries

A brief summary is presented of recent progress in estimating the rates of energy, momentum and mass transport of the solar wind through the magnetopause in terms of an analysis of the non-linear stage of various plasma instabilities. It is shown that the energy supply to the Earth's magnetosphere is due to reconnection on the dayside magnetopause and its dissipation during magnetospheric substorms, being controlled by both the interplanetary field parameters and by the dynamic pressure of the solar wind.     

Wave generation in the terrestrial magnetosphere

The Hard X-ray Imaging Spectrometer aboard the SMM detected gigantic arches in the corona which are formed or, if preexisting, become excited after major two-ribbon flares. They are seen in 3.5–8 keV X-rays and extend along the H _∥ = 0 line to altitudes between 10⁵ and 2 × 10⁵ km. These arches are stationary and form the base of a stationary type I radio noise storm initiated by the flare. They are visible in X-rays for ten hours or more and may be revived, in temperature, density, and brightness, if another two-ribbon flare appears below them. We suggest that they are built-up through reconnection process during the flare from the upper reconnected loops in the Kopp and Pneuman model. These loops become interconnected along the H _∥ = 0 line in consequence of great shear of the reconnecting loops. Obviously, the coronal transient associated with such flares must be either accomplished prior to the formation of the arch, or it must be formed through a process different from the Anzer-Kopp-Pneuman mechanism. Striking brightness variations occur quasi-periodically in the corona below and above the arch a few hours after the flare. These variations are seen at about the same time in soft X-rays, hard X-rays, and on centimeter microwaves in the low corona, as well as at metric waves in the type I noise-storm region. In spite of their flare-like intensity, however, the variations have little response in the transition layer (O v line) and no response at all in the chromosphere (Hα). We suggest that these semi-periodic brightenings are due to repetitive acceleration processes in plasmoids that encircle the arch perpendicular to the H _∥ = 0 line from the low corona through the noise storm region, being completely detached from the lower atmospheric layers.     

Large electric fields in the magnetosphere

In several regions of the magnetosphere, perpendicular and/or parallel electric fields are found to be orders-of-magnitude larger than expected from simple considerations. Problems associated with these large fields that may be amenable to study through computer simulations are discussed. Regions in which large electric fields are observed include: a) The auroral ionosphere, where Langmuir soliton-like structures have been measured to contain plasma frequency oscillations as large as 500 mV/m, the envelopes of which have parallel electric fields of 100 mV/m lasting for fractions of a millisecond; b) The auroral acceleration region, where electrostatic shocks have been observed to contain perpendicular fields as large as 1000 mV/m and parallel fields as large as 100 mV/m, and where double layers having parallel fields up to 10 mV/m have been observed; c) The high latitude boundary of the plasma sheet, where turbulent electric fields as large as 100 mV/m have been seen along with quasi-static fields of 5–10 mV/m; d) Inside the plasma sheet, where fields of 5–10 mV/m have frequently been observed; e) The bow shock, where turbulent fields as large as 100 mV/m and d.c. fields of 5 mV/m normal to the shock have been seen.also Physics Department     

Geomagnetic micropulsations and diagnostics of the magnetosphere

Plasma oscillations in a wide range of spectrum exist in the magnetosphere. Part of them penetrate the ionosphere and are recorded on the earth's surface. In the range of frequencies from millihertz to several hertz, the so-called micropulsations (ULF) are observed. In the range from hundred of hertz to several kilohertz the low-frequency emissions (VLF) are registered. Both types of emissions contain interesting and important information on the physical parameters of the magnetosphere and on the processes developing in it. The following paper describes the main problems of the diagnostics of the magnetosphere, which are based on the surface observations of micropulsations.In the first part of the paper, a short summary of theoretical conceptions on micropulsations is given. The main part of the paper describes the methods of diagnostics of the location of the boundary of the magnetosphere, of cold-plasma concentration in the outer regions of the magnetosphere, as well as of the energies and fluxes of fast charged particles in the geomagnetic trap. Some experimental results of the diagnostics of the parameters of the magnetosphere are given. Advantages and deficiencies of the existing methods of surface diagnostics are discussed, and the directions of further investigations are traced.     

Structure of reconnection layers in the magnetosphere

Magnetic reconnection can lead to the formation of observed boundary layers at the dayside magnetopause and in the nightside plasma sheet of the earth's magnetosphere. In this paper, the structure of these reconnection layers is studied by solving the one-dimensional Riemann problem for the evolution of a current sheet. Analytical method, resistive MHD simulations, and hybrid simulations are used. Based on the ideal MHD formulation, rotational discontinuities, slow shocks, slow expansion waves, and contact discontinuity are present in the dayside reconnection layer. Fast expansion waves are also present in the solution of the Riemann problem, but they quickly propagate out of the reconnection layer. Our study provides a coherent picture for the transition from the reconnection layer with two slow shocks in Petschek's model to the reconnection layer with a rotational discontinuity and a slow expansion wave in Levy et al's model. In the resistive MHD simulations, the rotational discontinuities are replaced by intermediate shocks or time-dependent intermediate shocks. In the hybrid simulations, the time-dependent intermediate shock quickly evolves to a steady rotational discontinuity, and the contact discontinuity does not exist. The magnetotail reconnection layer consists of two slow shocks. Hybrid simulations of slow shocks indicate that there exists a critical number,M _c, such that for slow shocks with an intermediate Mach numberM _IM _c, a large-amplitude rotational wavetrain is present in the downstream region. For slow shocks withM _I<M _c, the downstream wavetrain does not exist. Chaotic ion orbits in the downstream wave provide an efficient mechanism for ion heating and wave damping and explain the existence of the critical numberM _c in slow shocks.     

Electrodynamics of the magnetosphere: Geomagnetic storms

Geomagnetic and auroral storms provide a great deal of detailed information on the interaction between the solar plasma flows and the magnetosphere. Vast numbers of observations have been accumulated, and many theories have been developed to explain them. However, many of the most vital features of the interaction remain unsolved. The purpose of this paper is to provide the background for future work by summarizing fundamental morphological data and by reviewing critically the proposed theories.The paper consists of four sections. In the first section, the structure of the solar plasma flows and the magnetosphere are briefly discussed. Effects of the direct impact of the plasma flows on the magnetosphere are described in Section 2. Both Sections 3 and 4 are devoted to the discussion of the major phase of geomagnetic storms, namely the formation of the asymmetric ring current belt and the development of the auroral and polar magnetic substorms, respectively.Research supported in part by grants from the National Aeronautics and Space Administration to the University of Alaska (NsG 201-62) and to the University of Iowa (NsG 233-62).     

Interaction of the moon with the earth's magnetosphere

The general characteristics and dimensions of the earth's magnetosphere, turbulent transition region, and shock wave are reviewed and examined with regard to their possible active or passive interaction with the moon. The moon may, in turn, have a wake either detectable by space experiments outside the terrestrial shock wave or capable of interacting with the earth's shock wave, wake, or magnetospheric tail. The length and detectability of such a lunar wake depend, among other factors, on the selenomagnetic field and flow characteristics of the solar wind. Direct experimental information on a lunar wake is scanty, being, in fact, limited to one supposed case recorded by the IMP-1 satellite. Some evidence has been claimed, on the other hand, for statistical relationships between geomagnetic disturbance observed at the surface and the age of the moon. These results, however, are partly conflicting with each other as regards the phase of the supposed monthly variations, and partly are lacking statistical significance or merely grazing the significance level. The proximity of the lunar period with the length of the solar rotation, as well as a yearly and half-yearly modulation of the quasi-persistent 27-day fluctuations of geomagnetic disturbance contribute to encumber the question, rendering it very difficult, with the records presently available, to free the supposed lunar effects from solar interference. It seems, therefore, recommendable to postpone a judgement on the reality of such effects.Contribución del Instituto Antártico Argentino No. 109.